The State of Tuna Stock in TANZANIA
WHERE ARE WE?
Rushingisha George
TAFIRI
Masumbuko Semba
Mandela
Mathew Silas
DSFA
Yussuf Salim
ZAFIRI
2025-07-14
Agenda
- Introduction & Objectives
- Approach
- LBSPR Model
- Assumptions
- LW Data
- Stock Status by Species
- Overall Findings & Comparative Summary
- Management Recommendations & Conclusion
- Discussions
INTRODUCTION
Stock Assessment Report 2024
Stock Assessment Report 2025
Objectives
For Yellowfin, Bigeye, and Skipjack tuna.
- Estimate key biological and exploitation reference points (e.g., size at maturity, growth rates, mortality) for Yellowfin, Bigeye, and Skipjack tuna.
- Assess the current stock using the Length-Based Spawning Potential Ratio (LBSPR) model.
- Identify and map the spatial distribution of fishing intensity
Approach
Theorem- LBSPR Model
Length-Based Spawning Potential Ratio (LBSPR)
\[ \begin{align} SPR &= \frac{\text{Fished}}{\text{Unfished}} \\ &= \frac{\sum_{L} \phi(L) \cdot F(L)}{\sum_{L} \phi_0(L) \cdot F(L)} \end{align} \]
Assumptions
- Unit Stock: The populations are treated as single, self-contained stocks for this assessment.
- Representative Sampling: The data collected from the artisanal and industrial fisheries are representative.
- Equilibrium State: The fishery and the underlying population are assumed to be in a state of equilibrium.
- Closed Population: The stocks are considered demographically closed, with negligible immigration or emigration during the assessment period.
Length and Weight Data
Neritic waters
Data from the neritic zone were sourced from both TAFIRI and DSFA
EEZ
Data from Exclusive Economic Zone were sourced from DSFA
Fishing grounds
Sampling months
“The sampling spans all 12 months of 2024, ensuring comprehensive coverage. Data was collected consistently from January through December.”
Length and Weight Data…
Length Frequencies for 2024
Yellowfin tuna 
Skipjack tuna 
Bigeye tuna 
Reference points & Stock
Yellowfin Tuna (Thunnus albacares)
Juvenile and adult
Reference Points
Yield and Biomass
Summary
- SPR: 29%
- Indicates the stock is within the recommended range.
- This is an improvement over the 2024 report’s estimate of 13%, but still well below the 40% target.
- Critical Issue: Juvenile Catch
- Maturity Size (\(L_{50}\)): 92.3 cm
- Capture Size (\(SL_{50}\)): 59.5 cm
- This large gap shows that fishing pressure is heavily concentrated on immature fish, severely impacting the stock’s ability to replenish itself.
Skipjack Tuna (Katsuwonus pelamis)
Juvenile and adult
Reference Points
Yield and Biomass
Summary
- SPR: 33% (New Baseline)
- This is the first formal assessment for Skipjack in this series.
- The 33% SPR suggests the stock is within the sustainable level.
- Concern: Juvenile Catch
- Maturity Size (\(L_{50}\)): 59.9 cm
- Capture Size (\(SL_{50}\)): 50.2 cm
- Like the other species, a substantial portion of the catch consists of immature fish that have not had a chance to spawn.
Bigeye Tuna (Thunnus obesus)
Juvenile and adult
Reference Points
Yield and Biomass
Summary
- SPR: 33%
- Indicates the stock is within the sustainable fishing level.
- Represents a significant improvement in estimated status from the 2024 report’s 14% SPR.
- Concern: Juvenile Catch
- Maturity Size (\(L_{50}\)): 82.0 cm
- Capture Size (\(SL_{50}\)): 62.7 cm
- While less severe than for Yellowfin, the capture of immature individuals remains a key driver of stock pressure.
Key Finding
Overall Key Finding
All three assessed tuna stocks are within the recommended range of sustainable levels.
The Spawning Potential Ratio (SPR) for all species is above 20% but below the 40% target reference point.
The primary driver is the harvest of immature fish: The size at which fish are caught (\(SL_{50}\)) is consistently and significantly lower than the size at which they mature (\(L_{50}\)).
A Comparative Summary
The consistent gap between size at maturity (\(L_{50}\)) and size at capture (\(SL_{50}\)) is a major red flag.
| Species | Spawning Potential (SPR) | Size at Maturity (\(L_{50}\)) | Size at Capture (\(SL_{50}\)) | Status |
|---|---|---|---|---|
| Yellowfin | 29% | 92.3 cm | 59.5 cm | Sustainable |
| Bigeye | 33% | 82.0 cm | 62.7 cm | Sustainable |
| Skipjack | 33% | 59.9 cm | 50.2 cm | Sustainable |
An SPR range between 20% and 40% is the recommended.
Harvesting Immature Fish
The Core Problem: the average size at capture is significantly smaller than the size at maturity.